Is there a way to tell if an invoice was created from a memorized transaction?

If you want to use SuiteQL, there's a "memdoc" field on the Transaction table. It's the ID of the memorized transaction (which are stored in the "MemDoc" table).

Have to say I have to delve into SuiteQL but I guess this is a good reason to do so. Would it be possible to update a line item description via SuiteQL? That is my ultimate goal.

No. SuiteQL is only for reading.